1. Field of the Invention
This invention relates to pellets of copolymers of ethylene and an ethylenically unsaturated carboxylic acid, which have been surface-coated with a low level of certain liquid additives or water-soluble solid additives. The treated pellets have permanently increased block resistance which leads to improved pellet production rates and handling characteristics. Many of these additives are acceptable for food contact, allowing use of films made from copolymer pellets surface-coated with these additives in food packaging.
2. Description of Related Art
Ethylene copolymer pellets have an increasing tendency to `block`, that is to say clump together, at ambient or slightly elevated temperatures as the level of comonomer increases. However, different comonomers affect properties of ethylene copolymers containing them, including the tendency to block, to a differing extent for the same comonomer level, both on a weight and mole basis. There appears to be more than one cause of blocking. Thus, for instance, softness or flexibility which can be a significant causative factor for some copolymers to block, is not as important for other copolymers. In any event, when stick temperature, a measure of pellet blocking, is below 30.degree. C., resultant poor pellet handling characteristics during manufacture can affect production rates. This is partly because certain production operations such as monomer `purging` of polymer pellets, which uses hot air, become more efficient at higher temperatures. Increasing blocking during long storage can also become a significant problem for polymer processors.
Pellets of copolymers of ethylene with acrylic and methacrylic acid and certain ionomers derived from these copolymers are particularly prone to blocking. A common way of reducing blocking is by use of certain anti-blocking additives. However, these acid copolymers and their ionomers are used extensively in the food packaging industry, and when so used, the polymer must be acceptable to the FDA.
Many anti-blocking additives for ethylene copolymers, are known, but they may not all function in the same way. They may also be effective at quite different levels. Some additives are effective for many copolymers. Thus, U.S. Pat. No. Re. 32,325 (Smith) discloses use of certain solid bisamides to reduce blocking in a wide variety of ethylene copolymers, including ethylene/methacrylic acid copolymers as well as ethylene/vinyl acetate polymers and several other ethylene copolymers. The additive is incorporated while the polymer is molten, and must migrate to the surface of pellets to be effective. Rather high levels, preferably 2000-4000 ppm are required. These bisamides have limited FDA acceptability.
Other additives appear to be more specific to a given copolymer. U.S. Pat. No. 4,663,383 (Lowe et al.) discloses use of silica particles surface-coated on pellets at 50 ppm or more to reduce blocking. Polymers with no acid group are described as not showing significant improvement with such treatment. This additive may be added to solid dry pellets of the copolymer, or during cutting of the extruded polymer strand into pellets by mixing the additive with the melt cutter water. Since the additive is added to the surface, it is not required to migrate to the surface. While some forms of silica are acceptable for food contact use, silica has been found to cause equipment wear in polymer processing equipment. In addition, after mechanical handling of silica-coated pellets, the effectiveness of the silica in reducing blocking diminishes. Its ability to prevent blocking is thus not completely permanent.
U.S. Pat. No. 4,264,554 (Hale) discloses use of surfactants including ethylene oxide and propylene oxide homo and copolymers, preferably copolymers. The surfactants are added to water containing the pellets to reduce agglomeration of the pellets while they are in water, as they are when in the melt cutting water immediately after pelletization. Ethylene/vinyl acetate/methacrylic acid copolymers are mentioned. There is no indication these additives would prevent blocking of dry pellets.
In packaging applications, heat-sealability is a critical property. A useful additive to decrease blocking should not significantly affect the level of heat-sealability. Yet additives which reduce blocking at elevated temperatures can reduce heat-sealability. Silicone oil for instance, though an effective additive for increasing stick temperature, is known to adversely affect heat-sealability even at low levels.
Mechanical treatment, particularly some form of roughening of pellet surfaces also affects blocking. However, attempts to achieve surface modification in a controlled way has not been successful. In addition, such operations are time consuming, and could not be conveniently carried out at a stage in pellet production before which pellet blocking becomes a problem.
There is a need for an additive which will have FDA acceptability for use in contact with food, which can be readily surface-coated on polymer pellets to produce a permanent reduction in their blocking tendency, and which does not adversely affect heat-sealability or other utility of the polymer in a significant manner.